Windows 10 (later versions of windows 10) has a new feature where the default printer is changed to the last used printer every time you print. Initially I though this was a pain but I have started to embrace the new thinking, If I'm doing work that requires creating PDF's its convenient to have that "just work" until I change jobs to another scenario.

However if you don't want that behavior you can turn it off, click on the start button and type printers (see 1 and 2 below), click on printers & scanners (see 3 below)

Once you are viewing the printers & scanners section, scroll down and you will see the option for "let windows manage my default printer" (see 4 below) toggle it on or off as you like.